This invention relates to a method for removing a weak acid from an aqueous solution containing said acid. More particularly, this invention relates to the removal of such a weak acid with the use of an anion exchanger.
Netherlands patent application No. 241,033 teaches that a chloromethylated cross-linked polystyrene aminated with dimethylamine may be used to remove silica. The cross-linking reaction in the amination of the chloromethylated cross-linked polystyrene gives rise to the formation of quaternary ammonium groups with each quaternary group being attached to two benzene rings, in accordance with the following formula: ##STR1##
In the referenced Dutch patent application it is indicated that in the demineralization of aqueous solutions containing organic purities, the foregoing resin will be strongly fouled in repeated charging and regenerating cycles. Furthermore, it is believed that the fouling will lead to a large reduction in the strong base capacity of the resin, a less complete exchange or uptake of acids or anions, particularly of weakly dissociated acids such as silica, and the need for a larger volume of rinse water after regeneration. As a result of the foregoing, the resins may, after some time, become entirely useless. In the referenced Dutch patent application it is stated that the central problem is caused by the fact that on the one hand strongly basic groups can remove silica, but on the other hand, the fouling is directly related to the presence of such groups. Severe fouling problems are already said to arise in the case where the resin contains as little as 20% of strongly basic groups having the above-indicated structure.
In the Dutch patent application No. 241,033, the fouling problem is believed to be overcome by the use of macroporous, strongly basic resins containing at least 25%, by weight, and preferably more than 30%, by weight, of a cross-linking agent. A serious drawback to the resulting exchanger is that in spite of the macroporosity of the resin obtained, the product will become increasingly susceptible to fouling. In practice, such as resin will therefore still be insufficiently resistent to fouling to be commercially acceptable. Moreover, the amount of caustic soda used in the regeneration is particularly high as a result of the high degree of cross-linking and the resulting high selectivity to halide ions, relative to hydroxyl ions.